This invention relates to a method of preparing a structure having utility in the storage or transport of liquids at temperatures greatly differing from ambient temperature. The structure is primarily intended for enclosing cryogenic liquids, such as liquefied gases at about atmospheric pressure. It is used in conjunction with other materials such as aluminum for containers for cold liquids, for example liquefied gases, such as liquefied natural gas and more particularly liquid hydrogen, argon, propane and butane. Such containers are used, for example in marine tankers, for the transport of liquefied gases. A more detailed discussion of such containers is contained in a report to the Maritime Administration, U.S. Department of Commerce, entitled "LNG Containment Systems", May 1972, Contract MA-6562.
It is known that use of a hexagonal honeycomb wherein empty cells are filled with an insulating foam, provide a structurally intact cryogenic insulation having the utility heretofore disclosed; see U.S. Pat. No. 3,556,917, issued Feb. 14, 1966 to B. E. Eakin et al. (Class 161-44). Furthermore, U.S. Pat. No. 2,744,024, issued May 1, 1956 to H. A. Pace (Class 154-75) teaches the fabrication of such a honeycomb by the introduction of a foamable material into empty cells of the honeycomb and allowing the material to rise in the cells to fill them. Also properties of foams, such as polyurethanes, are known; see POLYURETHANES Chemistry, Technology and Properties, L. N. Phillips and D. B. V. Parker, London, Iliffe Books Ltd.
Also, U.S. Pat. No. 3,064,345, issued Nov. 20, 1962 to V. L. Herman et al. discloses immersing a honeycomb structure into a bath containing a heated liquid. The liquid consists of two layers. The top layer is a material which is solid at room temperature. It floats on a bottom layer which has a density greater than that of the material floating on top and which is a liquid at room temperature. After the structure, one open edge, settles to the bottom of the bath, the bath is allowed to cool to room temperature. Upon cooling the top layer solidifies against the edge surface of the honeycomb. After removal of the structure from the bath the honeycomb is modified in that a substantially rigid, airtight material is in contact with it. Because it is airtight a vacuum applied to one side of the hoenycomb holds it in place thereby permitting the honeycomb to be worked upon. After working the honeycomb the solid material, e.g., polyglycol, can be removed by contacting it with water. U.S. Pat. No. 3,555,131, issued Jan. 12, 1971, to V. P. Weismann, discloses placing an empty lattice in a form containing water. A quantity of membrane-forming liquid is poured onto the surface of the water. The liquid spreads over the water and forms a thin membrane. After the membrane is formed a desired amount of liquid foam material is added thereon. After the foam sets, the combination of lattice and solid foam are lifted from the form. The membrane adheres to the foam surface and acts as a moisture barrier. The resulting product is a reinforced modular foam panel. U.S. Pat. No. 3,274,322, issued Sept. 20, 1966, to J. S. Scudder, discloses placing a liquid layer of polyurethane onto a moving surface. The moving surface that contacts the polyurethane has on it an oily substance. The oily substance reduces sticking between the moving surface and the polyurethane. U.S. Pat. No. 3,187,069, issued June 1, 1975, to S. Pincus et al., discloses a method of making molded foamed articles in which a mold is lined with an easily removable plastic film. Liquid foam is placed into a mold so lined. The mold is closed until the expansion of the foam is complete. However, before curing of the foam is complete the mold is opened and the film surrounded foamed article is removed. The removed liner surrounded article is then heated at a curing temperature. A result of this method is that the mold itself is not heated to a deleterious curing temperature.
However, the aforementioned honeycomb insulation generally does not have the inherent tensile shear property to resist fracturing at cryogenic temperatures. The fracturing results from the thermal stresses imposed on the honeycomb from having a cryogenic liquid on one side and normal ambient conditions on the other side. Additionally, the side of the honeycomb not facing the cryogenic liquid does not provide a detection space when bonded directly to a supporting means. Therefore it does not offer any means for determining or monitoring the integrity of the primary skin against liquid intrusion.
Thus, in the practice of this invention, present method fabricates a honeycomb which overcomes the previously mentioned problems, i.e., the fracturing and lack of means for monitoring.